Bonding of poly(ethylene terephthalate) induced by low-temperature plasmas

ABSTRACT

Bonding of polyester tire cord to rubber by conventional adhesive treatments is improved by pretreating the yarn or the cord by brief exposure to a non-oxidizing low temperature gas plasma.

Unite States Patent 1191 Lawton Dec. 10, 1974 [5 BONDING OF POLY(ETHYLENE 3,305,430 2/1967 Hennemann 161/231 x TEREPHTHALATE) INDUCED BY 3,462,335 8/1969 Hansen et al. 156/272 3,526,583 9/1970 Hayward 204/165 LOW TEMPERATURE PLASMAS 3,647,676 3/1972 Bersin 204/165 X [75] Inventor: Ernest L. Lawton, Durham, NC. 3,661,735 5/1972 Drelich 204/165 3,690,926 9/1972 Wam etich et al 161/231 X Asslgneel M P "Y L0111S,M0- 3,723,289 3/1973 Boom 204/168 X [22] Filed: Feb. 14, 1972 [21] A p], No 225,764 Primary Examiner-Charles E. Van Horn Assistant Examiner-M. G. Wityshyn Art A t, F -,-Thom s Y. Awalt, J 521 11.5. c1 156/180, 156/272, 117/93.1 R, Omey gen a r 204/165 [51] Int. Cl B2911 9/02, BOlk 1/00 58 Field 01 Search 156/272, 180, 305, 117, [57] ABSTRACT 156/110 C; 117/93.3, 93.31, 93.1; 204/165, Bondin h g of polyester tire cord to rubber by conven- 161/231 250/531 tional adhesive treatments is improved by pretreating the yarn or the cord by brief exposure to a non- [56] References C'ted oxidizing low temperature gas plasma. UNITED STATES PATENTS r 3,057,792 10/1962 Frohlich 1. 204/165 6 Claims, l Drawing Figure CONTROL VALVE GAS TANK SEAL - -1 ACTIVATOR 1 RATOR PLATE 11 R F GENE ACTIVATOR TUBES PLASMA CHAMBER FEED AND TAKE- UP MECHAN'SM VACUUM PUMP (5 FT. MIN.)

VACUUM GAGE V VACUUM PUMP BLEED VALVE This invention relates to the service modification of polyethylene terephthalate fibers for the purposes of improving its bonding characteristics to rubber.

Extensive efforts have been made towards the improvement of the bonding characteristics of such textile materials as rayon, nylon and polyester, to polymeric materials because good adhesion is important to the manufacture of tires.

Adhesion between synthetic polymeric fibers had generally been accomplished by the use of resin-latex adhesives, such as resorcinolformaldhyde-latex adhesive (commonly known as RFL, or modifications thereof as described in U.S. Pat. Nos. 2,961,343; 2,561,215, and Re 23,451).

In the case of certain synthetic fibers, due to their suface characteristics, the resin-latex composition has generally not been satisfactory in holding the fiber side of the bind. In the case of polyester, other surface treating agents such as the isocyanate have been used in conjunction with the resin-latex composition, as a pretreatment for the fiber. Although this was satisfactory for the bonding of polyester tire cord or fabric to rubber, alternate methods were developed in order to avoid the use of the isocyanate. One of these alternates involved exposure to ionized gas. According to U.S. Pat. No. 3,477,902, the so-called glow discharge technique was used to activate gas for the pretreatment of fabrics, followed by conventional RFL bonding techniques. The glow discharge technique, and other hienergy discharges create an ionized gas which, when applied under certain-conditions to a fiber or filament, will penetrate and alter its basic characteristics.

It is an object of this invention to provide a method for use in conjunction with the conventional tire adhesion compositions, to bond polyester tire cord to rubher.

It is another object of this invention to provide a physical surface electron bombardment treatment which does not cause penetration of fiber to such an extent as its properties would be altered.

Yet another object of this invention is to provide an adhesive system for tire cord which permits lower curing temperatures for the adhesive without sacrificing the levels of adhesion obtained on untreated cord at higher temperatures of cure.

Briefly, the objects of this invention are attained in a process for bonding of polyester cords to rubber by exposing the polyester yarn to non-oxidizing low temperature gas plasma at a pressure of not more than about 2 torrs and a concentration of electrons of about 10 per cubic centimeters, for a period of at least about 1 second, before plying the yarn into cord. Without any further exposure of the cord to heat, the cord is then dipped in a resin-latex adhesive and subsequently cured. Alternatively, the yarn may be plied into cord, and the cord may then be treated in the low temperature plasma. As compared with chemical, thermal, or radiation modifications of polymer surfaces, the radio-frequency low-temperature plasma is unique in that the bulk of the polymer is not penetrated and bulk properties such as color, tensile strength, elongation, and dielectric constant are not altered.

In the description, reference will be made to the attached drawing in which the FIGURE. is a schematic representation of yarn or cord treating process.

In the. Figure, gas flows from the gas tank through the flow meter and control valve to the activator tubes. The generator is a 300 watt 13.56 M, radio frequency generator. There are no electrodes. Activator plates are in conjunction with a pyrex glass chamber and five activator tubes charge the gas. The gas employed may be active or inactive, but oxidizing plasmas such asoxygen "and air are least effective since they continually remove measurable quantities of material from the polymer surface. Thus the use of plasma such as oxygen may result in a weak surface layer containing partially oxidized polymer chains.

The low temperature" oxygen plasma consists of 10-20% atomic oxygen in a similar quantity of excited oxygen molecules. The oxygen molecules are excited from a triplet ground state to a spectroscopically excited first singlet. The following species have been identified in RF oxygen plasmas: 0+,0 +,0-, O ,O(3P), O Ag), and free electrons. The concentration of ions and electron will vary from 10 to 10 per cubic centimeter depending on the electric field strength, pressure, impurities (including reaction products such as CO from organic substrate) and wall ef fects. The concentration of the neutral species will vary from 10 to 10 per cubic centimeters. Ultraviolet emissions may also occur.

At a radio freqeuncey radiation of less than 1,000 Mc/ second in a gas that is in the l torr pressure range, the interaction will result in the ionized gas described above. Outside the region of activation, the ions will rapidly recombine with the electrons; but other species may persist. The translational temperature of neutral species in typical low-pressure plasma is approximately room temperature. The electrons within the lowtemperature plasma have sufficient kinetic energy for their temperature to be of the order of tens of thousands of K but their mass is too small for them to penetrate organic solids which they impinge on. The ionized species (about 0.01 percent of plasma) have translational vibration, and rotational temperatures that are slightly higher than neutral species due to their ability to extract energy directly from the electrical field. At higher pressures, temperature equilibrium is obtained between the electrons and atomic, molecular and ionic species resulting in the more familiar high temperature plasmas.

EXAMPLES Bundles of 1,000 denier/200 filament polyethylene terephthalate fibers were processed thorugh a commercially available Tracer Laboratories LTA-600 plasma generator, modified as shown in the drawing. Exposure to the plasma was under a pressure of 0.5:t0.l torr for a period of 9.4 seconds. F low rate of each gas was adjusted to achieve the 0.5 torr pressure in the plasma chamber. The flow rate varied from. 20 to 40 cubic centimeters per minute depending on the gas employed. After the fiber bundles were removed from the plasma chamber, they were twisted into a 2 ply (1,000 denier per ply) at 12 turns per inch Z twist end singles and 12 turns per inch S twist in the ply. Adhesives were applied as indicated and the rubber-to-cord adhesion was tested as indicated below. Three commercially available adhesives were employed. The first was a standard RFL dip prepared by dissolving 5.5 grams of resorcinol in l 16 milimeters of water containing 0.15 grams of sodium hydroxide. To this solution, 8.1 grams of 37 weight percent aqueous formaldehyde was added; and the solution was stirred for 5 minutes. This resin master solution was allowed to age for 6 hours at 7578F. The 133 grams of resin master was added to a dispersion of 122.0 grams of polyvinyl-pyridine-styrenebutadiene latex (41 weight percent solids) and 30.0 grams of deionized water, the resulting dispersion being gtixed for 15 minutes. The RFL dip was used within 72 ours.

Another dip consisting of a commercially available proprietary dip known as H7(lmperial Chemical lndu'stries Limited) was employed for purposes of comparison. This dip consisted of 82 milliliters of H-7 adhesive, 50 milliliters of polyvinyl-pyridine-styrene-butadiene latex (41 weight percent solids), and 72 milliliters of deionized water.

In each case the cord was dipped in the adhesive and passed through a 6 foot drying zone maintained at 165C, then into an electrically heated circulating hotair oven.

A third dip employed was the isocyanate-epoxy adhesive known as D417, (duPont) more particularly described in US. Pat. No. 3,307,966.

The cured cord was tested for static adhesion at TABLE 1 The peeling adhesion of 1 by 6 inch strips of a rubber cord composite [cord/adhesives/rubber (2 ply, 1,000/2 cord 48n/inch)]-was tested at 120C. The construction of these strip adhesion samples was in accordance with the modified strip adhesion method presented to the ASTM sub committee A-9 by .1. L. Brownlee on Oct. 17, 1962. A sample of rubber having 2 parallel plys of cord is pulled apart at a rate of 12 inches per minute 'at an angle of 180, and the peel force averaged for a 2 inch length of peel. Plys are constructed with the closest possible spacing of cords. Skim stock between plys was 15-20 mils.

The rubber used in H-block and strip samples reflected a Moony Scorch time (5 point rise/ 135C.) as 13.5 minutes. All samples had a cure time of 20 minutes at 153C, and had been subjected in block molds to 500 lbs. per square inch pressure at 300F. for 25 minutes.

Pull loads listed in the tables are average values from 8H-blocks. Peeling loads are average values from five strips from a single mold.

The following table illustrates the improvement in adhesion obtained by these treatments. 1t was discovered that so far as polyethylene terephthalate is concerned, the oxidizing plasmas, oxygen and air are the least effective, suggesting the possibility of actual oxidation of the surface layer of the fiber.

EFFECT OF GAS ON ADHESION IMPROVEMENT Adhesive Cure Conditions Strip Adhesion Peeling Load (lb/in.) at 120C.

N N A He NH Air 0 Control RFL 210C, 3 min. 16.9 15.3 14.5 11.5 21.0 7.5 2.5 RFL 232C, 3 min. 18.6 17.5 10.6 15.0 23.7 14.1 11.2 3.0 RFL 232C, 3 min. 17.0 2.0 H7 243C, 3 min. 19.0 16.0 D417 230C, 1 min.

204C, 1 min. 22.8 20.0

' Visual Estimate of of Cord Covered by Rubber Adhesive Cure Conditions After Peeling Test N N A He NH Air 0 Control RFL 210C, 3 min. 40 40 25 90 10 0 RFL 232C, 3 min. 10 50 98 20 40 O RFL 232C, 3 min. 0 H7 243C, 3 min. 98 D417 230C, 1 min.

204C, 1 min. 100 100 Adhesive Cure Conditions H-Block Pull Load (1b.) at C.

N N A He NH Air 0 Control RFL 210C, 3 min. 139:1.8 13111.9 15.6-10.7 11.7:1.1 9.5i09 10.6-10.7 7.5103 RFL 232C, 3 min. 17.0i1.8 166:1.8 13.8:12 13.9:fl19 10.7i1.7 11.4:t1.8 12.3fll6 8.6:07 RFL 232C, 3 min. 158:1.2 8.0-:12 H7 243C, 3 min. .7106 13511.3

120C. in the H-pull test as described in ASTM The above described plasma-treatment was applied D,-2138-67, except that three-sixteenths inch rather 55 to tire cords of 1,000 denier per ply/2 ply construction.

than one-quarter inch of cord was pulled from the rubber.

Cord was passed through the various plasmas at 1.030.14 pressure for a 9.4 second exposure time.

TABLE 2 PLASMA TREATMENT OF CORD Adhesive Cure Conditions Strip Adhesion Peeling Load (lb/in.) at 120C.

3,853,657 TABLE 2-Continued PLASMA TREATMENT OF CORD each exposure to oxygen plasma and nitrogen plasma l5 1. A treatment for filaments derived from a long is show chain synthetic polymer composed of at least 85 per- Bundles of 1,000 denier/200 filament polyethylene cent by welg ht Of 93 0f y ric 31601101 and terephthalate fibers were subjected to nitrogen plasma gphthahc acld. compnsmg exposmg the Surface of a at a pressure of 0.5 torr for varying exposure times at llber for a penod of atlleast about 1 Second a more a forward power level at 100 watts. Exposure time was OW temperature gas p asma at a pressure 0 not adjusted by controlling the takeup speed of the fiber 1? fgfilil o l g i z-g ag 2233 1232? zifgi ggl gf bundle Passing through the Plasma Chamber- The three bility of the filament to rubber with resin-latex adhetests reflect questionable adhesion improvement P sives is substantially improved without alteration of yond 4 seconds exposure tlme. There are two curing b lk properties of said filament,

temperatures shown. In each case the cord was held at 25 2. The process of claim 1 wherein the polyester filaconstant length for 3 minutes during curing. ments are in the form of filament bundles of a total de- TABLE 3 EFFECT OF EXPOSURE TIME TO PLASMA ON ADHESION IMPROVEMENT Exposure Time Strip Visual Estimate of of H-Block Pull Load Adhesion Peeling Load (sec) (1b./in.) Cord Covered by Rubber (1b.) at 120C.

at 120C.

. RFL 2l0C. RFL-230C. RFL-210C RFL-230C. RFL-210C. RFL-230C 0 2.5 3.0 0 0 7.5 .3 8.6;t0.7 4.3 22.3 22.9 90 98 10.7:1.7 15711.8 9.6 16.9 18.6 40 139:1.8 170:1.8 34.0 15.4 18.4 10 80 14.81118 104:1.5

Since better adhesion of the tire cord to rubber can nier of about SOD-1,5000 when treated with gas ordinarily be obtained by increasing the curing temperplasma. g ature of most adhesives, and since high curing tempera- 6 Process of Clalm wherein the polyester filarnres h a t d y t cause d d i f h l ments are m the form of tire cords when treated with ester cord, utilization of the process of this invention gas Plasma permits alowering of curing temperature without sacria prqcess f bonding COYQS to rubber, a fice of adhesion levels which would Otherwise be stanttal portion which cords consist of filaments detained for the untreated cord at the higher temperarived from a long Chain Synthetic polymer composed of at least 85 percent by weight of an ester of a dihydric tures This Illustrated by Table 4 below which Shows alcohol and terephthalic acid, wherein said individual for example, that equivalent adhesion can be obtained filaments in the form of yams are plied into a cord and Wlth the adhflswfi at temprature 3O lPwer by therafter dipped into a resin-latex adhesive and subsemeans of the application of the instant invention. q tly cured, the improvement comprising exposing TABLE4 EFFECT OF CURING TEMPERATURE AND TIME ON ADHESION IMPROVEMENT Temperature Time Strip Peeling Load (lb/in.) at 120C. Visual Estimate of of Cord Covered by Rubber Adhesive (C) (min; treated Control treated Control RFL 210 1 12.3 2.4 30 0 RFL 210 3 13.2 3.3 0 RFL 210 6 17.3 6.3 0 RFL 200 3 12.8 3.1 50 0 RFL 210 3 13.2 3.3 60 O RFL 220 3 15.1 3.9 80 0 RFL 230 3 16.7 5.0 70 0 H7 200 3 17.9 15.7 100 H7 230 3 20.1 17.9 100 100 30 1 22.8 20.0 100 100 the fibers are in the form of yarn when exposed to the gas plasma.

6. The process improvement of claim 4 wherein the fibers are in the form of cord when exposed to the gas UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,853,657

DATED December 10, 1974 INVENTOR(S) I E. L. Lawton It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1 line 5 "more" should read ---non-oxidizin line 6, "rnoe" should read ---more---;

Claim 2, line 3, "1,5000" Should read -1500:--.

sigr'aesi and serial tat-5.2: 10th June 1975' C. F-ULRSHALL DANN Cozr rtir:

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,853,657

DATED December 10, 1974 INVENTOR(S) E. L. Lawton It is certified that error appears in the ahove-identified patent and that said Letters Patent are hereby corrected as shown below:

C01. 4, Table 2, read I PAGE 2 TABLE 2 PLASMA TREATMENT OF CORD Adhesive Cure (onditiuns Strip Adhesion Peeling Load llbJin. at 120C.

2 2 (1)" (2) Control RFL 230C. 3 min. I [.6 5.0 H7 200C, 3 min. 30.8 15.7 H7 230C, 3 min. 16.8 19.6 20.4 17.9 RFL 230C, 3 min. l8 1 I60 l2.2

should read Lew PLASMA TREATMENT OF CORD Adhesive Cure Conditions Strip Adhesion Peeling Load (1b. /in. at 120 C.

N2 0 (1) (2)N Control RFL 230 c. 3 min. 11. s 5.0

H7 200C.. 3 min. 20.3 v 15. 7 i1? 23o c., a min. 16.8 19.6 2o. 4 i 17. 9

RFL 230C, 3 min. 18.3 16.0 12.2

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 53, 57 DATED December 10, 1974 INVENTOR(S) E. L. Lawton PAGE 3 It is certified that error appears in the aboveAdentified patent and that said Letters Patent are hereby corrected'as shown below:

Col. 5, Table 2 Continued, read TABLE 2-Continued PLASMA TREATMENT OF CORD Adhes'ue Cure Conditions Strip Adhesion Peeling Loud (Ill/in.) :it 120%. H7 300C. 3 min. 32.5 23.9 16.0

Visual Estimzn of 7 of and C w d b Adhesive Cure Cundiiiuns Ruhhe: ufrui Peen 1 Tutsi L y RFL 230C. 3 min. 50 2 H7 20071. 3 min. 100 I 100 H7 230C, 3 min 100 I I00 I00 RFL 230C. 3 min. 5 H7 200C, 3 min. I00 20 should read 11w Continued PLASMA TREATMENT OF com) Adhesive Cure Conditions Strip Adhesion Peeling Load (1b. limlgt C.

H7 200' C. 3 min. 100 100 20 

1. A TREATMENT FOR FILAMENTS DERIVED FROM A LONG CHAIN SYNTHETIC POLYMER COMPOSED OF AT LEAST 85 PERCENT BY WEIGHT OF AN ESTER OF DIHYDRIC ALCOHOL AND TEREPHTHALIC ACID COMPRISING EXPOSING THE SURFACE OF A FIBER FOR A PERIOD OF AT LEAST ABOUT 1 SECOND TO A MORE LOW TEMPERATURE GAS PLASMA AT A PRESSURE OF NOT MOE THAN ABOUT 2 TORR AND A CONCENTRATION OF ELECTRONS OF ABOUT 10**10-10**13 PER CUBIC CENTIMETER, WHEREBY BONDABILITY OF THE FILAMENT TO RUBBER WITH RESIN-LATEX ADHESIVES IS SUBSTAN-
 2. The process of claim 1 wherein the polyester filaments are in the form of filament bundles of a total denier of about 500-1, 5000 when treated with gas plasma.
 3. The process of claim 1 wherein the polyester filaments are in the form of tire cords when treated with gas plasma.
 4. In a process for bonding cords to rubber, a substantial portion which cords consist of filaments derived from a long chain synthetic polymer composed of at least 85 percent by weight of an ester of a dihydric alcohol and terephthalic acid, wherein said individual filaments in the form of yarns are plied into a cord and therafter dipped into a resin-latex adhesive and subsequently cured, the improvement comprising exposing the fibers to a non-oxidizing low temperature gas plasma at a pressure of not more than about 2 torr at a concentration of electrons of about 1010 -1013 per cubic centimeter for a period of at least about 1 second before dipping into said resin-latex adhesive.
 5. The processing improvement of claim 4 wherein the fibers are in the form of yarn when exposed to the gas plasma.
 6. The process improvement of claim 4 wherein the fibers are in the form of cord when exposed to the gas plasma. 